How to concat arrays immutable way JS

Question

I wonder how to contact array that is immutable. Lets imagine I start with array list = [4,1], and then I receive array from action response like so items = [5,2,6]. How do I concant arrays that the result is [4,1,5,2,6] and that operation is not mutable.

Bonus: How do I overwrite items with same id (immutable way)? Lets imagine this our array in store books=[{'id':1, 'title': 'Cool story'}, {'id':2, 'title': 'Bad story'}]. Other array that needs to overwrite books (last sync from API) otherArray = [{'id':3, 'title': 'Super story'}, {'id':1, 'title': 'Very cool story'}]. So result should be [{'id':2, 'title': 'Bad story'}], {'id':3, 'title': 'Super story'}, {'id':1, 'title': 'Very cool story'}]

"Bonus" No. Ask one question per question, not two. If you have a second question, ask it separately (after having done thorough research, of course). — T.J. Crowder
– T.J. Crowder, Commented Dec 18, 2016 at 15:08
"How do I overwrite items with same id (immutable way)?" You don't. You create a new array with a different value in one of the slots. — T.J. Crowder
– T.J. Crowder, Commented Dec 18, 2016 at 15:08

Max Cruz · Accepted Answer · 2018-10-21 18:10:45Z

15

With ES6 you can use destructuring:

const array1 = ["Banana","Apple"];
const array2 = ["Pineapple", "Peach"];
const array3 = [...array1, ...array2];

answered Oct 21, 2018 at 18:10

Max Cruz

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2 Comments

niltoid Over a year ago

this is better than the accepted answer for most modern use cases

Herbertusz Jun 10 at 13:05

This is not destructuring.

SLaks · Accepted Answer · 2016-12-18 15:06:52Z

7

Javascript does not have immutable types.

It sounds like you're actually asking to concatenate arrays without mutating the existing instances.

As stated clearly in the documentation, the concat() method does that.

answered Dec 18, 2016 at 15:06

SLaks

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1 Comment

Renato Lazaro Over a year ago

Number and String are immutables in Javascript. developer.mozilla.org/en-US/docs/Glossary/Mutable

Michael Lorton · Accepted Answer · 2016-12-18 15:16:24Z

1

Although as mentioned, the built-in method .concat() will solve your problem, you might wish to look into the Lodash library. In particular, the bonus question could be solved with _.unionBy(otherArray, books, "id").

answered Dec 18, 2016 at 15:16

Michael Lorton

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